The above described type of developing device is well known in the art in relation to a family of color image forming apparatuses typical of which are a color copier and a color printer. The developing device includes a plurality of developing units which are selectively operated to develop an electrostatic latent image provided on an image carrier in particular colors. The developing units are independent of each other and disposed around the image carrier. This increases the total number of structural elements of the device, resulting in the structure being complicated and the cost being increased. Further, since the developing units are mounted one by one to the housing of an image forming apparatus, high positional accuracy is not achievable without resorting to a complicated arrangement. In addition, adjustment involved in the inspection and maintenance of the individual units is troublesome.
All the developing units stated above may be constructed into a unitary assembly to promote common use of various structural elements of the different units and thereby to eliminate the above-discussed shortcomings. Such a scheme, however, brings about another problem, as will be described hereinafter.
A color copier, a color printer, or like color image forming apparatus usually has a capability of producing black-and-white copies by the black developing unit only as an ordinary black-and-white copier, in addition to a capability of producing color copies. Generally, this type of image forming apparatus is used more frequently for producing black-and-white copies than for producing color copies which may or may not include black. It follows that the rate of toner consumption in the black developing unit is higher than those in the other or color developing units. Further, when a developer is implemented as a two-component developer, the developer stored in the black developing unit becomes degraded earlier than those stored in the other developing units. For these reasons, the black developing unit usually needs maintenance more frequently than the color developing units.
In the above situation, if all of the color and black developing units are unseparably assembled together, the whole assembly has to be pulled out of the housing of the image forming apparatus when only one of them needs inspection, repair, or the like. Especially, when the black developing unit which is used more frequently than the color developing units needs some maintenance work, it is necessary to pull out the color developing units, which are still operable, along with the black unit. The result is troublesome and even wasteful manipulation.
As stated above, the independent developing unit scheme brings about a complicated structure and other various problems, while the unseparable assembly scheme obstructs easy and efficient maintenance.